Integrated circuits (ICs) are fabricated on substrates. Commonly, these substrates are semiconductor materials, and, in particular, silicon. As circuits increase in complexity, the number of transistors that are included in integrated circuits has increased. As complexity has increased, demand for smaller and more portable electronics has resulted in a conflict between the number of transistors in the integrated circuit and the size of the integrated circuit.
Transistor sizes have shrunk from, most recently, 65 nm to 45 nm, and will soon reach 32 nm. This reduction results in an increased density of the number of transistors in each unit of square area. Increasing transistor densities has allowed integrated circuits to continue to increase in complexity while also fitting in compact form factors, such as cellular phones. Currently, integrated circuits contain only one level of semiconductor dies. However, building multiple layers of semiconductor dies could increase the density of transistors several fold.
Stacked ICs are one route allowing continued expansion of the complexity of integrated circuits while reducing their size. Multiple substrates, each containing a layer of integrated circuits, may be stacked on top of each other resulting in a die with higher density of transistors in a given area than that of a single layer. Fabrication cost of integrated circuits is relative to the area consumed on a die, and hence, stacking two layers of integrated circuits in one die may be less expensive than building two separate dies.
In addition to transistors, integrated circuits include passive components such as capacitors. One challenge to designing integrated circuits is the placement of passive components around transistors in an efficient manner. As mentioned earlier, there is an economical advantage to reducing the amount of area consumed on a die. Still, there are areas of the die that are not used for active circuitry that could otherwise be used for passive structures.
Capacitors are used extensively in integrated circuits and several methods to incorporate them into the die exist. Such methods include soldering discrete elements onto either a packaging substrate or a board on which the die will eventually be mounted. Capacitive elements may also be built in the active circuitry. These methods all consume area on the die. It is advantageous to reduce the area consumed by passive structures as much as possible.
One structure with an extremely small footprint is the metal-insulator-metal (MIM) capacitor. Although offering a small footprint, the MIM capacitor still consumes area that otherwise could be used for active circuitry.
Thus, there is a need for a capacitive device that may be integrated into stacked ICs without consuming active area.